1. Field of the Invention
The present invention generally relates to an exposure system and a method for fabricating a true apodized fiber Bragg grating and, more particularly, to an exposure system and a method for fabricating a true apodized fiber Bragg grating by using two-beam interferometer.
2. Description of the Prior Art
When fabricating a complex fiber Bragg structure, refractive-index modulation and phase control are the most concerned parameters. It is therefore important to uniform the dc refractive-index modulation in addition to phase control during the exposure scan.
The Taiwan Patent Publication No. 436,667 discloses a method for fabricating a low-noise fiber Bragg grating by using a phase mask and an aperture so as to achieve uniform dc refractive-index modulation. Firstly in the method, a UV light passes through the aperture and then the phase mask as to form a grating in a photosensitive fiber. Secondly, the phase mask is moved backwards for a certain distance so that a non-interfered area (in the included angle of the first order diffracted light) serves as compensation of the refractive index of the fiber Bragg grating. Similarly, the U.S. Pat. No. 5,830,622 discloses a two-step method for modulating the variation of the refractive index. Firstly in the method, a grating is formed in the fiber. Secondly, light with pre-determined intensity distribution is irradiated on the grating. The above-referenced method is simple but requires a two-step exposure process. Moreover, the relation between the variation of the refractive index of the fiber and the light intensity for exposure is non-linear, resulting in critical control in light intensity in the second step exposure. As a matter of fact, these two prior art patents disclose methods that can not be immune from non-uniform refractive index and therefore the undesired noise.
In the United States Patent Publication No. 20020015919, there is therefore provided a method overcoming the afore-mentioned problems. In the method, two light beams with perpendicular polarization directions of electric fields are provided. An interference fringe pattern is formed behind a phase mask after the light beams are incident on the phase mask by an incident angle. Accordingly, the interference fringe pattern is adjusted by tuning the included angle of the light beams as well as the distance between the fiber and the grating so as to achieve modulation of the refractive index. Even though this method overcomes the problems brought forth by the Taiwan Patent Publication No. 436,667 and the U.S. Pat. No. 5,830,622, the phase difference in the interference fringe is very sensitive to precise control in the included angle between the light beams and the distance between the fiber and the phase mask. Moreover, the U.S. Pat. No. 5,367,588 discloses a grating with uniform dc refractive-index modulation by using a specially designed phase mask. The phase mask has a period-variable grating as to compensate the variation of the refractive index of the fiber. Even though the phase mask is more convenient in use than the other prior art patents, it suffers from the high price. In addition, due to the lack of flexibility in designing the variation of the refractive index of the grating, different phase masks are required when various fiber gratings are to be exposed.
European Patent No. 1065535 discloses an apparatus including a light-enhancement screen mask in addition to a phase mask for exposure in the fiber. When both of the two screen masks are irradiated by the same light beam, the light intensity on different grating positions remains constant during the exposure scan. However, the application of this method is limited due to standard design of the light-enhancement screen mask, which lacks of changeability in modulation of refractive index.
Furthermore, a perturbation method can also be used. However, it requires an interferometer to precisely control the displacement of the fiber corresponding to the mask, which is very sensitive to the perturbation of the exposure system. On the other hand, when the conventional two-beam interferometer is used for exposure, the reflection mirrors corresponding to the two light beams are required to be precisely controlled to achieve precise exposure. However, it is difficult to do so.
Therefore, the present invention has been made to solve the afore-mentioned problems in view of the forgoing status and to further provide an exposure system and a method for fabricating a true apodized fiber Bragg grating by using two-beam interferometer.